In an image-capturing device such as a video camera or a digital camera or the like which employs an image sensor, an image which is imaged by a lens is converted into an electrical signal by the image sensor. At this time, if a high frequency component which is at the same frequency level or higher than the sampling frequency of the image sensor is included in the image of the object to be photographed, then the phenomenon of deterioration of the photographic image may occur, which is undesirable. Due to this, an optical low pass filter is used with this type of image-capturing device, with the objective of attenuating high frequencies in the image formed of the object to be photographed.
An optical low pass filter is a device which, by separating a beam of light from the object to be photographed into a plurality of beams, attempts to eliminate the image of the high frequency component therein; and, generally, the method is employed of taking advantage of the birefringence character of a crystalline material to separate the light beam into an ordinary beam and an extraordinary beam. Quartz crystal or Chile saltpeter or the like is known as a crystalline material which is endowed with the birefringence character (for example, refer to Patent Reference #1). Furthermore, instead of employing a crystalline material which is endowed with the birefringence character, sometimes the effect of diffraction by a minute periodic structure may also be utilized.
Moreover, although with the per se known type of optical low pass filter described above the frequency characteristic is fixed, such a device has been proposed in which it is possible to alter the frequency characteristic (for example, refer to Patent Reference #2). With this optical low pass filter, the birefringence character which is generated by applying a mechanical stress to an optical material is employed, so that the light beam from the object to be photographed is separated into a plurality of beams.
Patent Reference #1: Japanese Laid-Open Patent Publication 2002-122813.
Patent Reference #2: Japanese Laid-Open Patent Publication 2003-167123.